I used to conduct seminars on using computers in the work place. People tend not to realize, or, if they DO realize it, to ignore this simple fact:
The company owns the network, the server and (usually) the computers that are on the company's network. That means ANYTHING (i.e. ALL activity) that is ON that network--emails, Internet searches, downloads, Facebook visits, and online game-playing--belongs to the company. They are entitled to look at every email, track an employee's time at shopping sites, or access anything that come INTO or goes OUT ON their network. Even if you are accessing the company network from your home computer or laptop or phone, anything that goes into the company network or to a company email on the network belongs to the company.
This also includes the ownership of laptops or phones that the company provides to its employees. They may tell you, "It's yours to use on the job, and we don't care if you take it home and use it for your personal use at home when you aren't working," etc., but in the end, they still own that laptop and everything on it.
With this in mind, your friend used the company email to share personal information and communicate non-business-related information with you. His "hitting on you" email belongs to the company. If there is a company policy that employees receive or sign off on that reminds them they are to use the company technology only for company business, then he is in what George W. Bush and his VP Dick Cheney called "Deep Doo-Doo." He can be suspended or even fired. LOTS of companies do absolutely NOTHING about the "abuse" of their networks, and employees get used to the company's laissez-faire attitude. But if the company decides they want to find something on a given employee to ease him/her out the door legally, the "violation of computer policy" abuses often take a front seat. There is just no argument against it if you're the one who has been "caught."
So... if you really want him to stop, send him an email on the company network, something like, "Bob, I just want to remind you that we are not supposed to use the company's network for personal communications. Please don't send me any personal correspondence. Thanks."
If you want to send him that email and then drop off a piece of paper with your phone number or with your personal email address on it, (and a reminder not to send you anything from the company email or by using the company server or network), that's different.
Just make sure you keep it all off the official company network.